The present invention relates to a method for operating an SCR catalytic converter for the aftertreatment of exhaust gases of an internal combustion engine, wherein a heatable reducing agent tank for the required liquid reducing agent is provided, and the tank heater has PTC characteristics. Additional items of the present invention are a computer program and a computer program product, which are suited for carrying out the method.
Methods and devices for operating an internal combustion engine are known, in whose exhaust gas region an SCR catalytic converter (Selective Catalytic Reduction) is disposed, which reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. In so doing, the nitrogen oxides in the exhaust gas can be considerably reduced. Nitrogen oxides (NO, NO2) are thereby preferably reduced while undesirable side reactions are largely suppressed. Ammonia (NH3), which is added to the exhaust gas, is needed for the reaction process. As a rule, NH3 detaching reagents are used as the reducing agent. An aqueous urea solution is often used.
The required aqueous urea solution (HWL) for the exhaust gas aftertreatment is held available in a reducing agent tank and is sprayed into the exhaust gas tract dependent on requirements via a reducing agent metering device. Depending upon the anti-freeze added, the aqueous urea solution freezes at −11 E C or lower. A tank heater is provided as a rule to thaw the aqueous urea solution. Provided that the aqueous urea solution is completely frozen, the system start-up can be delayed in order to thaw a required amount of the urea solution in this time. Provided that a temperature is detected in the tank, which is lower than the freezing threshold, the tank heater is activated so that at least a minimum amount of the aqueous urea solution can be thawed. Ohmic heating elements as well as PTC elements are conventionally installed in a tank heater. PTC elements are electrical resistors with temperature dependent behavior. The resistance of a PTC element increases with rising temperatures so that the current flow is reduced. It is hereby assured that a pronounced overheating or a continuous current feed does not occur.
A problem with a reducing agent tank with a completely frozen urea solution is that the output introduced by the tank heater during a relatively short driving cycle, for example less than two to three hours, can only thaw a limited volume of the reducing agent around the heater. A portion of said volume is extracted via the reducing agent metering device and is sprayed into the exhaust gas tract for the exhaust gas aftertreatment. A cavity, in particular a gas bubble, respectively air bubble, forms in the region of the tank heater as a result of the extraction of the liquid reducing agent. Provided that the urea solution subsequently refreezes and this process repeatedly occurs, the cavity enlarges so that the tank heater is eventually for the most part surrounded by gas, respectively air. As a result, the heat flow in the reducing agent tank greatly decreases on account of the air insulation and no longer can a sufficient amount of reducing agent be thawed. The system pressure breaks down and a metering of the reducing agent into the exhaust gas tract is no longer possible. This leads as a rule to an error message relevant to the OBD (on-board-diagnosis) system.
The aim behind the invention is to provide a solution to this problem and to assure the operability of the SCR catalytic converter even at low temperatures.
This aim is met by a method for operating an SCR catalytic converter.